Optical metrology system for spectral imaging of a sample

ABSTRACT

An optical metrology device is capable of detection of any combination of photoluminescence light, specular reflection of broadband light, and scattered light from a line across the width of a sample. The metrology device includes a first light source that produces a first illumination line on the sample. A scanning system may be used to scan an illumination spot across the sample to form the illumination line. A detector spectrally images the photoluminescence light emitted along the illumination line. Additionally, a broadband illumination source may be used to produce a second illumination line on the sample, where the detector spectrally images specular reflection of the broadband illumination along the second illumination line. The detector may also image scattered light from the first illumination line. The illumination lines may be scanned across the sample so that all positions on the sample may be measured.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. Ser. No. 14/091,199, filedNov. 26, 2013, which is assigned to the assignee hereof and which isincorporated herein by reference.

BACKGROUND

Photoluminescence imaging and spectroscopy is a contactless,nondestructive method of probing the electronic structure of materials,such as silicon semiconductor wafers, solar cells, as well as otherworkpieces and materials. In a typical photoluminescence process, lightis directed onto a wafer or other workpiece (hereinafter collectivelyreferred to as a “sample”), where at least some of the light isabsorbed. The absorbed light imparts excess energy into the material viaa process of “photo-excitation.” The excess energy is dissipated by thesample through a series of pathways; one such pathway is the emission oflight, or photoluminescence. The intensity and spectral content of thephotoluminescence is directly related to various material properties ofthe sample and, thus, can be used to determine certain characteristicsof the sample, including defects, as discussed in U.S. Pat. No.7,113,276B1, which is incorporated herein by reference.

Reflectance or reflectivity imaging is a contactless, nondestructivemethod of probing the surface with a broadband illumination source andanalyzing the intensity and spectral content of the signal bounced backfrom the surface. The surfaces typically can be classified into specularor diffuse surfaces and real objects typically exhibit a mixture of bothproperties.

It is sometimes desirable, e.g., for semiconductor wafer inspectionapplications, to measure intensity and spectral content of thephotoluminescence and reflectance of the semiconductor wafer-sizeworkpiece for the purpose of quality inspection in the same apparatuseither concurrently or in a short sequence, with single wafer load,while achieving a high measurement throughput combined with highmeasurement spatial and spectral resolution.

Conventionally, spectral photoluminescence or combined spectralphotoluminescence and reflectance are measured using a singlepoint-by-point inspection solution. In a point-by-point solution, thesample is placed on an X-Y motion (or R-Θ) system and is illuminated andmeasured at a single excitation point. The sample is moved to anothermeasurement point and again illuminated and measured. By repeating thetranslation of the sample in the X-Y directions, a photoluminescence andreflectance maps could be constructed from the point-by-pointmeasurements. This solution, however, is inherently slow and thereforeimpractical in the full wafer inspection systems, especially at largespecimen sizes, close to and above 100 mm in diameter, due to the lowthroughput.

SUMMARY

An optical metrology device is capable of detection of any combinationof photoluminescence light, specular reflection of broadband light, andscattered light from a line across the width of a sample. The metrologydevice includes a first light source that produces a first illuminationline on the sample. A scanning system may be used to scan anillumination spot across the sample to form the illumination line. Adetector spectrally images the photoluminescence light emitted along theillumination line. Additionally, a broadband illumination source may beused to produce a second illumination line on the sample, where thedetector spectrally images specular reflection of the broadbandillumination along the second illumination line. The detector may alsoimage scattered light from the first illumination line. The illuminationlines may be scanned across the sample so that all positions on thesample may be measured.

In one embodiment, an apparatus includes a light source that produces anillumination beam; a optical system that receives the illumination beamand produces an illumination spot on a surface of the sample; a scanningsystem that scans the illumination spot to form an illumination lineacross the sample, wherein the scanning system scans the illuminationbeam in a plane that is at a non-normal angle of incidence on thesample, and wherein the sample emits photoluminescence light in responseto excitation caused by the illumination spot along the illuminationline; a stage for providing relative movement between the illuminationline and the sample; a detector that images the photoluminescence lightemitted along the illumination line on a two-dimensional array with afirst dimension representing spatial information along the illuminationline and a second dimension represent spectral information of thephotoluminescence light, wherein the detector produces an image framerepresenting the photoluminescence light emitted along from theillumination line, and wherein the detector produces a plurality ofimage frames for the illumination line on the surface of the sample asthe stage produces relative movement between the illumination line andthe sample; and a processor coupled to the detector to receive theplurality of image frames and generates a photoluminescence image of thesurface of the sample.

In one embodiment, an apparatus includes a stage for providing relativemovement between the illumination line and the sample in a firstdirection; a first illumination source that produces a first light beam;a first lens system that causes the first light beam to be incident on asurface of the sample as a first illumination line orientated along asecond direction that is different than the first direction, the firstillumination line being incident on the sample at a first angle ofincidence, wherein the sample emits photoluminescence light in responseto excitation caused by the first light beam along the firstillumination line; a broadband illumination source that producesbroadband light; a second lens system that focuses the broadband lightonto the sample as a second illumination line orientated along thesecond direction and that is overlaid on the first illumination line onthe surface of the sample, the broadband light being incident on thesample at a second angle of incidence that is different than the firstangle of incidence; a detector that receives reflected broadband lightfrom the surface of the sample from the second illumination line andreceives the photoluminescence light emitted by the sample along thefirst illumination line, wherein the detector images the reflectedbroadband light and the photoluminescence light on a two-dimensionalarray with a first dimension representing spatial information along thesecond direction and a second dimension representing spectralinformation of the reflected broadband light and the photoluminescencelight and produces an image frame in response, wherein the detectorproduces a plurality of image frames for a plurality of positions of thefirst illumination line and the second illumination line that areoverlaid on the surface of the sample as the stage moves the sample inthe first direction; and a processor coupled to the detector to receivethe plurality of image frames and stores the plurality of image framesas a three dimensional data cube with two dimensions representingspatial information of the surface of the sample and a third dimensionrepresenting spectral information.

In one embodiment, a method includes illuminating a surface of a sampleat a first angle of incidence with a first light source along a firstillumination line having an orientation in a first direction, whereinthe sample emits photoluminescence light from the first illuminationline in response to excitation caused by light from the first lightsource; illuminating the surface of the sample at a second angle ofincidence with a second light source along a second illumination linehaving an orientation in the first direction and that overlays the firstillumination line, wherein the second angle of incidence is differentthan the first angle of incidence, and wherein the second light sourceis a broadband light source, wherein broadband light is reflected fromthe surface of the sample; detecting the photoluminescence light emittedby the sample along the first illumination line and specular reflectionof broadband light from the second illumination line with atwo-dimensional array having a first dimension representing spatialinformation corresponding to position along the first illumination lineand the second illumination line and a second dimension representingspectral information; moving the first illumination line and the secondillumination line across the surface of the sample in a second directionthat is different than the first direction; and producing a threedimensional data cube with two dimensions representing spatialinformation of the surface of the sample and a third dimensionrepresenting spectral information using detected photoluminescence lightand detected specular reflection of broadband light as the firstillumination line and the second illumination line are moved across thesurface of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical metrology device capable of simultaneousdetection of any combination of photoluminescence light, specularreflection of broadband light, and scattered light from a line acrossthe width of a sample.

FIG. 2A illustrates a top view of the surface of the sample with anillumination spot from a first light source that is scanned across thewidth of the sample to produce an illumination line and the illuminationline is moved across the sample.

FIG. 2B illustrates a top view of the sample with the illumination linefrom a second light source and the illumination line is moved across thesample.

FIGS. 3A and 3B illustrate a perspective view and side view,respectively, of a dark channel observation of scattered light by theoptical metrology device of FIG. 1.

FIGS. 4A and 4B illustrate a perspective view and side view,respectively, of the bright field reflectance observation by the opticalmetrology device of FIG. 1, with a narrow band light source andassociated optics omitted for clarity.

FIGS. 5A and 5B illustrate a perspective view and side view,respectively, of the excitation of the sample with the scanningillumination beam and the emitted photoluminescence light collection bythe optical metrology device of FIG. 1.

FIG. 6 illustrates an incident illumination beam and the Lambertiancharacteristic of the emitted photoluminescence light.

FIGS. 7 and 8 illustrate a side view (along the Y-Z plane) and a frontview (along the X-Z plane), respectively, illustrating the simultaneouscollection of dark field scattered radiation, bright field reflectanceradiation and photoluminescence light by the optical metrology device ofFIG. 1.

FIG. 9 illustrates the signal separation of the three spectral channels(dark field scattered radiation, bright field reflectance radiation, andphotoluminescence light) imaged by a 2D sensor array.

FIG. 10 is a flow chart illustrating a method of optical metrology datafrom a number of light sources.

FIGS. 11 and 12 illustrate side views (along the Y-Z plane) ofalternative configurations of the optical metrology device from FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an optical metrology device 100 capable ofsimultaneous detection of any combination of photoluminescence light,specular reflection of broadband light, and scattered light from a lineacross the width of a sample. The sample is moved in a single directionto sweep the line across the sample so that data may be quicklycollected from every position on the surface of the sample.

The metrology device 100 includes a first light source 110, which maybe, e.g., a narrow band illumination source, such as a laser. By way ofexample, the light source 110 may be a high intensity laser, such as aContinuous Wave (CW) laser with peak wavelength at 405 nm and power in 1mW to 500 mW range, depending on the photoluminescence efficiency of themeasured sample and desired signal intensity to be recorded by thedetector. If desired, more than one laser may be used to producemultiple narrow band wavelengths that are combined for light source 110.By way of example, other laser wavelengths, such as 266 nm, 355 nm, 375nm, 532 nm, 640 nm or 830 nm, and others not listed here, mayadditionally or alternatively be used, either individually orselectively combined. Laser(s) used as light source 110 may operateeither in Continuous Wave or Q-Switched mode of operation. If theQ-Switched (QS) laser is used for sample excitation, the instantaneouspower, i.e. power during the pulse, may be much higher, e.g., in a few(2.5 kW) kilowatt range.

A lens system including optics 112 is used to produce an illuminationspot with the illumination beam 114 on the surface of the sample 101.The illumination spot produced by the illumination beam 114 should havea size and/or power density to excite photoluminescence in the sample101. By way of example, the illumination spot size may be between 50 μmto 1 mm range and/or have a power density of between approximately 0.1W/cm² to 10⁸ W/cm² range. For example, if a CW 1 mW laser is focused toabout a 1 mm spot, the power density is about 0.127 W/cm². The same CW 1mW laser focused to 50 micron spot will give power of 50 W/cm². If ahigher power 500 mW CW laser is used, and is focused to a 1 mm spot, apower density of 63 W/cm² is reached and the same laser focused to 50 μmwill lead to the power density of 2.5*10⁴ W/cm². Thus, typical powerdensities for CW lasers are in 0.1 W/cm² to 2.5*10⁴ W/cm² range. Withthe use of Q-Switched lasers for sample excitation, the power densitiesare much different. For example, at an average power of 1 mW, and pulseduration of 10 nanoseconds (10*10⁻⁹ s) and repetition rate of 100 kHz,the momentary power may be as high as 1 W and the corresponding powerdensity at a 1 mm spot is 127 W/cm². When the same laser is used with anaverage power of 500 mW, and is focused to 50 μm spot size, themomentary power may reach, e.g., 2.5*10⁷ W/cm2.

The lens system may further include a scanning system 116, illustratedas including a scanning mirror 118 and a fixed mirror 120, that is usedto scan the illumination spot across the width of the surface of thesample 101 as an illumination line 122, that is illustrated as beingorientated along the X direction. The scanning speed/frequency ofscanning system 116 may be adjusted depending on the scan resolution andsensor read-out speed. For example, the frequency may be, e.g., between50-100 Hz range, but may vary in a range of 1 Hz to 10 kHz or more. Thescanning minor 118 moves to scan the illumination spot across thesurface of the sample 101 and may be, e.g., a (swinging) galvanometricmirror or a rotating polygonal minor. FIG. 2A, by way of example,illustrates a top view of the surface of the sample 101 with anillumination spot 124, produced by illumination beam 114 (FIG. 1) thatis scanned across the width of the sample, as indicated by arrows 125.As illustrated, by lines 121 in FIG. 1, the scanning system 116 scansthe illumination beam 114 across the sample 101 within a plane, whichcreates a non-zero angle of incidence α1, with respect to surface normalN. Thus, the illumination line 122 is incident on the surface of thesample 101 at a non-normal angle of incidence. It should be understoodthat the illumination line 122 is additionally scanned across thesurface of the sample 101 by the stage 104 (FIG. 1) moving the sample101 along the Y direction, as illustrated by arrow 123, so that theillumination line 122 may be incident on all positions on the surface ofthe sample 101.

It should be understood that FIG. 1 illustrates one configuration of thenarrow band illumination source 110 and associated optics, includingscanning system 116, but that other configurations may be used ifdesired. For example, while the use of fixed minor 120 is advantageousto simplify system alignment, if desired, the fixed mirror 120 may beremoved from scanning system 116, with the light source 110, lens 112,and scanning minor 118 repositioned so that the illumination beam 114illuminates the sample 101 directly, without the need for theredirecting mirror 120. Moreover, it should be understood that theoptics 112 may focus the illumination beam 114 into an illumination spot124 on a surface of the sample 101. If the illumination beam 114 isfocused at a sample plane, which is coincident with the surface of thesample 101, the illumination spot may become slightly defocused as theillumination beam moves along the illumination line 122, but the signalintensity variations caused by the defocusing can be compensated bysignal processing within computer 150. Alternatively, by way of example,the optics 112 may collimate the illumination beam 114. If desired, thecollimated illumination beam 114 may be focused on the surface of thesample using an optional F-theta lens 117, shown with dotted lines inFIG. 1. Moreover, if desired, the collimated illumination beam 114 maybe incident on the surface of the sample 101 as the illumination spotwithout being focused, with an associated loss of resolution andexcitation conditions.

By scanning the illumination spot 124 across the sample 101 to producethe illumination line 122, a high power density of the incident lightmay be maintained. Consequently, the illumination beam from light source110 imparts energy into the material of the sample via“photo-excitation” thereby producing photoluminescence light emittedfrom the sample along the illumination line 122. Additionally, surfacedefects on the sample 101, such as scratches, particles, epitaxialgrowth defects, e.g., stacking faults or mounds, may scatter theillumination beam 114 as it is scanned along the illumination line 122.

As illustrated, by lines 121 in FIG. 1, the scanning system 116 scansthe illumination beam 114 across the sample 101 within a plane, whichcreates a non-zero angle of incidence α1, with respect to surface normalN. Thus, the illumination line 122 is incident on the surface of thesample 101 at a non-normal angle of incidence.

The optical metrology device 100 includes a detector 130 that receiveslight from the surface of the sample 101 along a detector path 131 thathas a non-zero angle α3 with respect to surface normal N. Thus, asillustrated in FIG. 1, the detector 130 has viewing angle α3 that isdifferent than the angle of incidence α1 of the illumination line 122.As a result, the specular reflection of the illumination beam 114 fromthe surface of the sample 101 does not enter the detector path 131.Typically, the photoluminescence signal is a few orders weaker than thereflected radiation signal. With the use of the angle of incidence α1and the viewing angle α3, the optical metrology device 100 permits theillumination beam 114 to reflect from the specular surface of the sample101 without interfering with the photoluminescence signal received bythe detector 130, thereby avoiding the need for filtering the reflectionof the illumination beam 114. Moreover, because the illumination beam114 is not filtered, the detector 130 may receive light scattered bysurface defects without receiving illumination beam 114, i.e., enablingdark field observations. It should be understood that thephotoluminescence light produced by the sample 101 in response toexcitation by the illumination beam 114 will have a differentwavelength(s) than the illumination beam 114 itself. Accordingly, thewavelengths from the photoluminescence light and the scattered light canbe dispersed into different spectral channels. Consequently, the opticalmetrology device 100 may be used for simultaneous detection of surfacetopographical defects and photoluminescence. Additionally, with the useof the scanning system 116, the light source 110 may function as a lineillumination source without loss of power density or illuminationuniformity along the line if a cylindrical lens or cylindrical minorwere used.

The detector 130 includes optics 132, a spectrometer 134, and a sensor136 that includes a two-dimensional CCD or CMOS sensor array. The lightcollected along the detector path 131, e.g., photoluminescence light orscattered light, is collected by the fore optics 132, then passesthrough a narrow entrance slit aperture in the spectrometer 134. Thefield of view of the spectrometer 134 is limited by the entrance slit,which matches the orientation of the illumination line 122. Thus, theentrance slit of the detector 130, or to be more exact, the spectrometer134, is aligned with the illumination line 122 and overlaying broadbandillumination line 142 (discussed below), i.e. the entrance slit andillumination lines 122 and 142 all belong to the same plane, whileillumination line 122 and 142 are overlaid on top of each other and theentrance slit to the detector 130 is parallel to the illumination lines122 and 142. The spectrometer 134 disperses the spectrum of the receivedlight and the sensor 136 at the exit of the spectrometer 134 recordswith a two dimensional (2D) sensor array and produces a resulting imageframe, with a first dimension of the sensor array representing thespatial position along the illumination line 122 and the seconddimension of the sensor array representing spectral information. Thespectrometer 134 separates the wavelengths of emitted photoluminescencelight and separates the scattered light along one dimension of the 2Dsensor array, while the position along the illumination line 122 isrecorded by the second dimension of the 2D sensor array. For example,one point along the illumination line 122 may emit a maximumphotoluminescence at 460.3 nm, while another point on the illuminationline 122 may emit a maximum photoluminescence at 460.8 nm. Thus, thespectrometer 134 separates the wavelengths of the emittedphotoluminescence light to perform spectral photoluminescence imaging.

The metrology device 100 further includes a second light source 140,which may be, e.g., a broadband illumination source, such as a halogenlight source, that includes wavelengths of light that differ from thewavelengths used by the first light source 110 or the wavelengths ofphotoluminescence light emitted by the sample 101 in response toexcitation by the illumination beam 114. The broadband radiation source(sometimes referred to as a “white” light source) is formed into theillumination line 142 which is aligned with and overlays theillumination line 122 on the surface of the sample 101. As illustrated,the illumination line 142 may be produced, e.g., using a series ofoptical fibers 144 (only one of which is illustrated as coupled to thelight source 140). By way of example, the second light source 140 withthe series of optical fibers 144 may be a Lightline product manufacturedby Schott North America, Inc. The light from the multiple optical fibers144 is formed into a nearly collimated line-like beam with a cylindricallens 146. FIG. 2B illustrates a top view of the sample 101 with theillumination line 142 from the second light source 140.

As illustrated in FIG. 1, the broadband light from light source 140illuminates the surface of the sample 101 along a plain, illustrated bylines 141 that is at a non-zero angle of incidence α2, with respect tosurface normal N. Thus, the illumination line 142 is incident on thesurface of the sample 101 at a non-normal angle of incidence α2. Itshould be understood that the illumination line 142 is additionallyscanned across the surface of the sample 101 (along with overlaidillumination line 122) by stage 104 (FIG. 1), as illustrated by arrow145 in FIG. 2B, so that the illumination line 142 may be incident on allpositions on the surface of the sample 101. The angle of incidence α2 isdifferent from the angle of incidence α1 and has a sign that isdifferent from the sign of angle α1, i.e., the illumination line 142 isincident on the surface of the sample from an opposite direction as theillumination line 122, and has a different angle of incidence. The angleof incidence α2 of the illumination line 142, however, is equal, butopposite in value, as the viewing angle α3 of the detector path 131,i.e., viewing angle α2=−α3. Thus, the detector 130 viewing angle istuned to the incident angle of the illumination line 142 and as aresult, the specular reflection of the broadband light alongillumination line 142 is also aligned with the field-of-view of thespectrometer 134. Thus, both the illumination line 122 produced by lightsource 110 and the illumination line 142 produced by the light source140 are received by the detector. The spectrometer 134 separates thewavelengths of the specular reflection of the broadband illuminationalong the illumination line 142 into one dimension of the 2D sensorarray, while the position along the illumination line 142 (and theillumination line 122) is represented by the second dimension of the 2Dsensor array.

The broadband light source 140 may use wavelengths of light that aredifferent than the wavelength(s) used by the first light source 110 andwavelength(s) of the photoluminescence light emitted by the sample 101so that the spectrometer 134 may separate the wavelengths from thereflected broadband light from the wavelengths of the scattered lightand the wavelengths of the photoluminescence lights. Accordingly, thefirst light source 110 and second light source 140 may be used with thedetector 130 to simultaneously detect the spectral information withrespect to position along with illumination lines 122 and 142 for thephotoluminescence light caused by the excitation of illumination beam114, the dark field scattering of the illumination beam 114, as well asthe bright field reflectance from the light source 140. By way ofexample, the optical metrology device 100 may use a range of wavelengthsbetween 400-1,000 nm (i.e. 600 nm range), based on the wavelengths ofthe narrow band light source 110, the broadband light source 140, andthe emitted photoluminescence light. The detector 130 may separate thereceived light into, e.g., 1200 wavelengths, i.e., number of pixels inthe spectral dimension of the sensor array, and thus, the detector 130may have a spectral resolution of 0.5 nm. Of course, if desired, otherspectral resolutions may be used, as well as wavelengths of light orranges of wavelengths of light, as well as the number of wavelengthsdetected by detector 130.

Moreover, the broadband light that is specularly reflected from thesurface of the sample 101 is directed to the detector 130 without anyneed for mechanical repositioning of the detector 130, therefore thedetector 130 can collect the surface reflectance, scattering andphotoluminescence signals concurrently or in a quick succession withoutany delay for mechanical repositioning of any apparatus opticssubcomponents. Of course, if desired, the first light source 110 andsecond light source 140 may be used in quick succession so that thedetector 130 does not simultaneously receive light from bothillumination lines 122 and 142.

The sample 101 is held on a linear stage 104 that can translate thesample 101 in a direction that is different than the orientation of theillumination lines 122 and 142. For example, the orientation of theillumination lines 122 and 142 may be in a direction (e.g., theX-direction) that is orthogonal to the direction of travel of the linearstate 104 (e.g., the Y-direction). The stage 104 translates the sample101 to place the illumination lines 122 and 142 at multiple positionsacross the sample 101 (as illustrated by arrows 123 and 145 in FIGS. 2Aand 2B) and the spectral imaging of the illumination lines 122 and 142is repeated at each new position. The process of imaging and moving thesample 101 is repeated to scan the illumination lines 122 and 142 acrossthe sample 101 thereby producing a series of 2D image frames. Ifdesired, the stage 104 may move the sample 101 in steps or move thesample 101 continuously so that data acquisition is performedcontinuously (e.g., with a high frequency scan of the illumination spot124), without the Y-axis motion stopping at each line. For example, whena high frequency scan (e.g., 500 Hz) of the illumination spot 124 isused with a relatively low speed stage motion along the Y-axis and lowspeed collection of image frames (e.g., 100 Hz), the illumination beam114 is scanned over the illumination line 122 several times for eachgiven image frame, e.g., five times in the given example, which providesfor improved signal averaging. The speed of the stage motion along Yaxis may be expressed in millimeters per second and depends on thedesired resolution along the Y direction. For example, when the Y speedis 20 mm/s and the frame rate is 100 frames per second, the Y resolutionis 20 mm/s divided by 100/s equals 0.2 mm.

Thus, in one data capture operation, the optical metrology device 100 isable to collect concurrently the spectral photoluminescence and spectralscattered radiation and spectral reflected radiation signals from theline-illuminated portion of the sample 101, and may move in a singleaxis and repeatedly perform the data capture operation to acquire datafor the entire sample surface. In one embodiment the data for the entiresample surface is obtained by moving the sample 101 underneath of theillumination lines 122 and 142 with a linear stage in the Y-direction.In another embodiment, however, the data can be collected by rotatingthe sample 101 underneath of the illumination lines 122 and 142 with arotary stage in Θ (angle) direction, illustrated in FIG. 1 with a dottedarrow. In both embodiments, the sample 101 is moved in one axis only,either Y or Θ (both are not required), resulting with a high speedmeasurement. In comparison, conventional systems acquire data from asingle illumination spot and must move the sample in two axes to acquiredata for the entire sample surface. Thus, the optical metrology device100 uses a single unidirectional stage, as opposed to the conventionaltwo linear or linear and rotary stage systems. Moreover, dataacquisition is accelerated because of the elimination of required stagemotion along the X-axis.

It should be understood the motion between the illumination lines 122and 142 and sample 101 is relative, and thus, if desired, the stage 101may be held stationary and the illumination lines 122 and 142 may bemoved (laterally in the Y direction or rotated in the Θ direction usinga stage to move, e.g., the light sources and associated optics withrespect to the sample 101, or other appropriate means.

The plurality of image frames produced by the detector 130 as the sample101 is moved and spectral information from the line-illuminated portionsof the sample 101 is acquired may be received by a computer 150, whichmay store the plurality of image frames as three dimensional (3D) datacube. The 3D data cube includes two dimensions that are spatial (e.g.,one dimension is the position along the illumination lines 122 and 142(X axis) and the other dimension is the position of the line scannedacross the sample (Y axis)) and a third dimension represents thespectral information. The detector 130 is coupled to provide the imagedata to the computer 150, which includes a processor 152 with memory154, as well as a user interface including e.g., a display 158 and inputdevices 160. A non-transitory computer-usable medium 162 havingcomputer-readable program code embodied may be used by the computer 150for causing the processor to control the metrology device 100 and toperform the functions including the analysis described herein. The datastructures and software code for automatically implementing one or moreacts described in this detailed description can be implemented by one ofordinary skill in the art in light of the present disclosure and stored,e.g., on a computer readable storage medium 162, which may be any deviceor medium that can store code and/or data for use by a computer systemsuch as processor 152. The computer-usable medium 162 may be, but is notlimited to, magnetic and optical storage devices such as disk drives,magnetic tape, compact discs, and DVDs (digital versatile discs ordigital video discs). A communication port 164 may also be used toreceive instructions that are used to program the computer 150 toperform any one or more of the functions described herein and mayrepresent any type of communication connection, such as to the internetor any other computer network. Additionally, the functions describedherein may be embodied in whole or in part within the circuitry of anapplication specific integrated circuit (ASIC) or a programmable logicdevice (PLD), and the functions may be embodied in a computerunderstandable descriptor language which may be used to create an ASICor PLD that operates as herein described.

By way of example, the computer 150 may use the photoluminescencesignals received from the detector 130 for each position on the surfaceof the sample 101 as stored in the 3D data cube and generate aphotoluminescence image (or map) of the sample 101. Thephotoluminescence image may be, e.g., a map of signal intensity of thephotoluminescence signal. Inspection of the photoluminescence intensityimage may be used for process control to assure that all portions of thesample 101 meet desired specifications. For example, where the sample101 contains manufactured light emitting diodes (LEDs) chips, inspectionof the photoluminescence data, e.g., in the form of a photoluminescenceintensity image, can be used to assure each LED will have appropriatebrightness. Similarly, the photoluminescence intensity image may be usedfor defect segmentation and predicting yield losses based on presence oflocalized low photoluminescence signals, which can lead toout-of-specification device at the Back-End of Line. If desired, thephotoluminescence signal may be processed to produce other images ormaps. For example, a Peak Lambda image may be produced to show thedistribution of Peak Lambda over the sample surface, where Peak Lambdais the wavelength at which any given point in the image or map hasmaximum photoluminescence. Thus, for example, one point on the samplesurface may emit maximum photoluminescence at 460.3 nm, while anotherpoint on the surface of the same sample may emit a maximumphotoluminescence at 460.8 nm, which can be clearly seen with a PeakLambda image. With the use of the spectral photoluminescence imaging,the different wavelengths of photoluminescence light emitted by thesample may be identified. Accordingly, the optical metrology device 100may be used for process control to assure that all points on the samplesurface emit photoluminescence within a predefined wavelength range.Additionally or alternatively, the photoluminescence signal can betransformed into a Full-Width-Half Maximum (FWHM) image, which shows theFWHM value for each Peak Lambda at any given point on the sample. TheFWHM image, by way of example, may be used to assure that light emittingdiodes (LEDs) that are manufactured with the sample emit light withinpredefined-width (band) spectral range. The photoluminescence signal maybe processed to produce images of the sample 101 other than thephotoluminescence intensity, Peak Lambda and FWHM images. For example,the photoluminescence signals may be processed or analyzed to producedifferent qualities, such as, e.g., a map of photoluminescence intensityat a given fixed wavelength, which is different than the maximumphotoluminescence intensity map. Moreover, images or maps of interestmay be produced by combining sets of images, such as those discussed,e.g., by pixel-by-pixel multiplication. Thus, the photoluminescencesignal may be processed to produce other desired images of the sample101 based on the recorded photoluminescence signals or analyzed in otherways for process control during manufacture of the sample.

Additionally, the computer 150 may process the received reflectedbroadband signal to determine a characteristic of the sample 101 atmultiple positions and produce a map of that characteristic. Forexample, layer thickness may be calculated based on the spectralresponse associated with reflection of the broadband light, and thus,the received reflected broadband signal may be used to determinethickness for points on the sample surface and an epilayer thicknessimage (or map) may be produced. Thus, the optical metrology device 100may be used to monitor the epitaxial layer thickness at any given pointon the surface of the sample, which may be used to assure that themeasured thickness is within predefined range for a given epitaxialgrowth process. Additionally, using the received scattered light, thecomputer 150 may produce a darkfield image of the surface of the sample101, thereby exposing surface defects, which may be related scratches,particles, epitaxial growth defects, e.g., stacking faults or mounds,etc.

FIGS. 3A and 3B illustrate a perspective view and side view,respectively, of the dark channel observation of scattered light byoptical metrology device 100. As illustrated, the light source 110produces illumination line 122 on the surface of the sample 101 usingthe optics 112 and scanning system 116, including scanning mirror 118and fixed mirror 120. If the sample were defect free, light would bespecularly reflected by the surface of the sample 101, as illustrated bylines 125, and would not be detected by the camera sensor 136. When asurface defect 126 is present on the sample 101, a portion of theillumination beam 114 is scattered when the illumination beam 114 isscanned over the defect 126. The scattered light along the detector path131 received by the detector 130 and enters the spectrometer 134 throughthe slit aperture. The field of view of the spectrometer 134 is limitedby a slit aperture, and therefore, the sensor 136 only images theillumination line 122. The specular reflection of the illumination beam114 is not received by the spectrometer 134. The spectrometer 134normally separates light into wavelength bins in the spectral dimensionof the 2D sensor array. The scattered light, however, originates fromthe narrow band light source 110 and, thus, the scattered received bydetector 130 is deflected by the spectrometer 134 into the bin(s)corresponding to the wavelength(s) of the light source 110, which isrecorded by the sensor 136. The performance of the scattered lightchannel may be further altered by adding light polarizer and analyzerinto the optical path of the narrow band illumination beam 114. By wayof example, a beam polarizer 119 may optionally be placed downstream ofthe light source 110, e.g., between the lens 112 and minor 118, and theanalyzer 133 may be combined with the fore optics 132.

FIGS. 4A and 4B illustrate a perspective view and side view,respectively, of the bright field reflectance observation by opticalmetrology device 100, omitting the light source 110 and associatedoptics for clarity. The broadband light source 140 illuminates thesample 101 along the illumination line 142 line extending across thewidth of the sample 101 along an orientation that matches theorientation of the entrance slit of the spectrometer 134. As a result,the light from the light source 140 that is specularly reflected fromthe surface of the sample 101 enters the spectrometer 134 through theslit aperture. Within the spectrometer 134, the broadband light isseparated (binned) into a series of wavelengths, which are recorded by2D sensor array in the sensor 136, with one dimension of the 2D sensorarray representing the spectral information and the other dimensionrepresenting the spatial information along the illumination line 142.The arrangement of the broadband light source 140, thus, is a brightfield mode of operation in which an illumination line 142, which may bethe width of the sample 101, is spectrally imaged. The use of anon-normal angle of incidence for the bright field observationadvantageous simplifies uniform illumination and spectroscopic imagingof the sample 101 along illumination line 142.

FIGS. 5A and 5B illustrate a perspective view and side view,respectively, of the excitation of the sample 101 with the scanningillumination beam 114 and the emitted photoluminescence light collectionby optical metrology device 100. Similar to FIGS. 3A and 3B above, thelight source 110 produces illumination line 122 on the surface of thesample 101 using the optics 112 and scanning system 116, which mayinclude scanning mirror 118 and fixed mirror 120. As discussed above,fixed minor 120 may be removed or replaced and/or a F-Theta lens may beused. The light from illumination beam 114 that is specularly reflectedis illustrated by lines 125. Another portion of the light fromillumination beam 114, however, enters the sample 101 and is absorbed.The absorbed energy generates electron-hole pairs, which uponrecombination emit photoluminescence light along the excitation line,i.e., illumination line 122. The generated photoluminescence lightemitted from the excitation line exits the sample 101 at multipledirections with near Lambertian characteristics. FIG. 6, by way ofexample, illustrates the incident illumination beam 114 and theLambertian characteristic of the emitted photoluminescence light (thespecularly reflected light is not illustrated in FIG. 6). In addition,because the illumination beam 114 is scanned across the width of thesample 101 along illumination line 122, the photoluminescence light isemitted along the excitation direction, as illustrated in FIG. 5A. Theemitted photoluminescence light along the detector path 131 is receivedthrough the slit aperture of the spectrometer 134, where the light isdeflected by the spectrometer 134 into the bin(s) corresponding to thewavelength(s) of the photoluminescence light, which is recorded by thesensor 136.

FIGS. 7 and 8 illustrate a side view (along the Y-Z plane) and a frontview (along the X-Z plane), respectively, illustrating the simultaneouscollection of dark field scattered radiation, bright field reflectanceradiation and photoluminescence light by the optical metrology device100. As illustrated, the illumination beam 114 generates scattered light115, and excites photoluminescence light 117, that are received by thespectrometer 134, along with the reflected broadband light 143.

FIG. 9, by way of example, illustrates the signal separation of thethree spectral channels (dark field scattered radiation, bright fieldreflectance radiation, and photoluminescence light) imaged by the 2Dsensor array of sensor detector 136. As illustrated, the 2D sensor arrayincludes a first dimension (the X axis in FIG. 9) representing thespatial information along the illumination lines 122 and 142, with theother dimension representing the spectral information, i.e., wavelengthλ, with respect to the signal intensity. In FIG. 9, the scattered laserbeam signal is illustrated by peaks 202, the photoluminescence signal isillustrated by peaks 204, and the reflected broadband beam signal isillustrated by peaks 206.

Thus, as illustrated in FIGS. 7, 8, and 9, the detector 130 cansimultaneously record the photoluminescence light generated in thesample 101 in response to excitation by the illumination beam 114 and abright field reflection of the broadband light 143. If desired, thedetector 130 may further record a dark field scattering of theillumination beam 114 by defects on the sample so that, if desired,three separate signals are simultaneously imaged. The three signals maybe recorded simultaneously along the entire width of the sample 101 in amacroscopic mode, as opposed to being recorded point-by point or withina narrow field of view in a microscopic as is conventionally performed.Moreover, one detector 130 is used to collect all three signals, so thatthere is no spatial or time shifts between the three signals. Aspectrometer 134 separates the signals into three different spectralinformation channels, i.e., signals are spectrally separated that arerecorded in different wavelength bins (sensor pixels) with the 2D sensorarray sensor of the detector 130. It should be understood that the imageframe illustrated in FIG. 9 is for one position of the illuminationlines 122 and 142 along the Y axis in FIG. 1 and that as the stage 104moves the sample 101 in the Y direction to scan the illumination lines122 and 142 across the sample, the sensor 136 will produce a pluralityof image frames for each position of the illumination lines 122 and 142along the Y axis in FIG. 1.

FIG. 10 is a flow chart illustrating a method of optical metrology datafrom a number of light sources. As illustrated, a surface of a sample isilluminated at a first angle of incidence with a first light sourcealong a first illumination line having an orientation in a firstdirection (302). The sample emits photoluminescence light from the firstillumination line in response to excitation caused by light from thefirst light source. By way of example, as discussed above, the samplemay be illuminated with an illumination beam produced, e.g., using anarrow band light source 110, and that is scanned across the sample toproduce the first illumination line. The surface of the sample isilluminated at a second angle of incidence with a second light sourcealong a second illumination line having an orientation in the firstdirection and that overlays the first illumination line (304). Thesecond angle of incidence is different than the first angle ofincidence, and the second light source is a broadband light source. Thebroadband light is reflected from the surface of the sample. Thephotoluminescence light emitted by the sample along the firstillumination line and specular reflection of broadband light from thesecond illumination line is detected with a two-dimensional array havinga first dimension representing spatial information corresponding toposition along the first illumination line and the second illuminationline and a second dimension representing spectral information (306). Thespecular reflection of light from the first light source along the firstillumination line may not be detected. The first illumination line andthe second illumination line, which are overlaid, are moved across thesurface of the sample in a second direction that is different than thefirst direction (308). For example, the first direction and seconddirection may be orthogonal. Movement of the first illumination line andthe second illumination line may be caused by a linear stage or a rotarystage moving the sample 101 with respect to the illumination lines. Ifdesired, movement of the first illumination line and the secondillumination line may be caused by a stage moving the first illuminationline and the second illumination line with respect to the sample 101,e.g., by moving the light sources and associated optics with respect tothe sample 101 which may be held stationary. A three dimensional datacube with two dimensions representing spatial information of the surfaceof the sample and a third dimension representing spectral information isproduced using detected photoluminescence light and detected specularreflection of broadband light as the first illumination line and thesecond illumination line are moved across the surface of the sample(310).

Additionally, as discussed above, a portion of the light from the firstlight source may be scattered off surface defects on the sample asscattered light and the scattered light may be detected with thetwo-dimensional array, where the three dimensional data cubeadditionally includes the detected scattered light. Using the scatteredlight in the three dimensional data cube, a surface defect image of thesurface of the sample may be produced. Additionally, a photoluminescenceimage of the surface of the sample may be produced using the detectedphotoluminescence light in the three dimensional data cube. For example,the photoluminescence image of the surface of the sample may be, e.g., aphotoluminescence intensity image, photoluminescence Peak Lambda image,or a photoluminescence Full-Width-Half Maximum (FWHM) image.Additionally, a characteristic of the sample may be determined for aplurality of positions on the surface of the sample using the detectedspecular reflection of broadband light in the three dimensional datacube, and an image of the surface of the sample may be generated usingthe characteristic of the sample for the plurality of positions. Forexample, the epilayer thickness of the sample 101 may be determined at aplurality of locations on the surface of the sample 101 and an epilayerthickness image (or map) may be produced. As discussed above, the datafrom the three dimensional data cube may be analyzed for process controlduring manufacture of the sample.

FIG. 11, by way of example, illustrates a side view of an opticalmetrology device 100′, that is similar to the optical metrology device100 illustrated in FIG. 7, except that the narrow band illumination beam114′ is illustrated as being incident from the same side of normal N asthe broadband illumination 141. Thus, while the narrow band illuminationbeam 114′ and broadband illumination 141 are incident on the surface ofthe sample 101 from the same side of normal N, they use different anglesof incidence, so that the specular reflection 125′ of the narrow bandillumination 114′ is not along the detector path 131, thereby enablingdark field mode measurement. As can be seen in FIG. 11, the angle ofincidence of the illumination beam 114′ is greater than the angle ofincidence of the broadband illumination 141, respect to normal N, butthis is not strictly required. FIG. 12, by way of example, is similar toFIG. 11, and illustrates an optical metrology device 100″ that isconfigured so that the narrow band illumination beam 114″ has a smallerangle of incidence compared to the broadband illumination 141, respectto normal N, while the specular reflection 125″ of the narrow bandillumination 114″ is still not along the detector path 131, therebyenabling dark field mode measurement.

Although specific embodiments are provided herein for instructionalpurposes, the described embodiments are not limiting. Variousadaptations and modifications may be made without departing from thescope of the present discloser. For example, a rotary stage may be usedin place of a linear stage for scanning the illumination lines 122 and142 across the surface of the sample 101. Moreover, the scanning system116 may be modified to eliminate, e.g., the fixed mirror, or an F-thetalens may be used for focusing the illumination beam 114 on the surfaceof the sample 101. Other modifications and variations are possible, andtherefore, the spirit and scope of the appended claims should not belimited to the foregoing description.

What is claimed is:
 1. An apparatus comprising: a light source thatproduces an illumination beam; a optical system that receives theillumination beam and produces an illumination spot on a surface of thesample; a scanning system that scans the illumination spot to form anillumination line across the sample, wherein the scanning system scansthe illumination beam in a plane that is at a non-normal angle ofincidence on the sample, and wherein the sample emits photoluminescencelight in response to excitation caused by the illumination spot alongthe illumination line; a stage for providing relative movement betweenthe illumination line and the sample; a detector that receives thephotoluminescence light emitted along the illumination line on an arraythat represents spectral information of the photoluminescence light, andwherein the detector produces a plurality of spectral information withrespect to spatial position for the illumination line on the surface ofthe sample as the stage produces relative movement between theillumination line and the sample; and a processor coupled to thedetector to receive the plurality of spectral information with respectto spatial position and characterizes the sample based on the spectralinformation with respect to spatial position.